cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/ipc/begin_frame_args.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module cc.mojom;
 
 import "mojo/common/time.mojom";
 
 enum BeginFrameArgsType {
   INVALID,
   NORMAL,
   MISSED,
   BEGIN_FRAME_ARGS_TYPE_MAX
 };
 
 // See cc/output/begin_frame_args.h.
 struct BeginFrameArgs {
+  uint64 source_id;
+  uint64 sequence_number;
   mojo.common.mojom.TimeTicks frame_time;
   mojo.common.mojom.TimeTicks deadline;
   mojo.common.mojom.TimeDelta interval;
   BeginFrameArgsType type;
   bool on_critical_path;
 };
+
+// See cc/output/begin_frame_args.h.
+struct BeginFrameAck {
+  uint64 source_id;
+  uint64 sequence_number;
+  uint64 latest_confirmed_frame;
+  uint32 remaining_frames;
+  bool has_damage;
+};